Automatically create groups that do not exist

If the scheduler hint 'group' is specified and the contents are not a
UUID, automatically create the group.  This is to maintain backwards
compatibility with the old behavior of the 'group' scheduler hint.

The policy on this automatically created group is set as 'legacy'.
That way the scheduler knows that for this group it should emulate the
old group behavior for backwards compat purposes.

Partial-bug: #1296913

Change-Id: I0d4315980af7e47ae97dd4277784a74f086c8825
Merge "Automatically create groups that do not exist"